This application claims the benefit of Japanese Patent Application No. 09-48563, filed in Japan on Feb. 18, 1997, and No. 09-215464, filed in Japan on Jul. 28, 1997, both of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a backlight device and, more particularly, to a backlight device for use in liquid crystal display devices and the like.
2. Discussion of the Related Art
Recently, liquid crystal display (LCD) devices having a backlight system are becoming increasingly popular. The LCD devices modulate polarized light obtained by transmitting light through a polarizing plate through a liquid crystal layer.
FIG. 11 shows the typical structure of a conventional backlight system and an LCD device. Light emitted from light source 21 is incident on light guide 22 and propagates by undergoing multiple total internal reflections. A portion of the light traveling through the light guide 22 is diffused by light diffusers 23 and is emitted from the light guide. A portion of the light emitted downward from the light guide 22 is reflected by reflection sheet 24 and returned to the interior of the light guide 22. The light emitted upward from the light guide 22 is diffused by diffusion sheet 25, and is converged by prism sheet (or lens sheet) 26 to enter liquid crystal cell 30 interposed between polarizing plates 31 and 32.
FIGS. 12 and 13 show other types of conventional LCD devices having a backlight system.
The backlight device of FIG. 12 is structured such that the order of diffusion sheet 25 and prism sheet 26 is reversed from that of FIG. 11 and the prism surface of the prism sheet is facing the light guide 22.
The backlight of FIG. 13 is structured such that an additional prism sheet 26a, which has a plurality of triangular column prisms arranged in a direction perpendicular to the arrangement direction of the triangular column prisms in prism sheet 26, is placed over the prism the sheet 26.
However, currently used light polarizing plates absorb approximately half of the incident light, and thus have low efficiency in light usage. That is, in order to have sufficient luminance, more light needs be incident on the light polarizing plate. However, increasing the light intensity of the light source causes a variety of problems, such as increased power consumption of the light source and adverse effects on the liquid crystal due to heat generated from the light source, which degrade the display quality.
To solve these problems, the following techniques have been proposed. A polarization light splitter is provided for splitting unpolarized light from a light source into two linearly polarized light beams having the polarization directions orthogonal to each other. One of the split polarized light beams is directly used for illumination while the other polarized light beam is used indirectly. In other words, these techniques improve light usage efficiency such that, of the two linear polarization components of light, one component is incident on the liquid crystal cell, and the other component is reflected by the polarization light splitter, returned to the light source side, and is reflected again to reintroduce it to the polarization light splitter. Some of the recent developments along this direction are as follows.
(1) In Japanese Laid-Open Patent Application No. 04-184429, an unpolarized light beam from a light source is split by a polarization light splitter into two linearly polarized light beams having polarization directions orthogonal to each other. One of the polarized light beams is emitted directly towards the liquid crystal cell. The other is returned to the light source side, converged, and then is reflected for re-use as light for illuminating the liquid crystal cell.
(2) A backlight system disclosed in Japanese Laid-Open Patent Application No. 06-265892 has a beam deflector disposed over a planar light guide to direct light emitted from the planar light in a direction approximately perpendicular to the surface of the planar light guide. A polarization light splitter is located over the beam deflector.
(3) A backlight system disclosed in Japanese Laid-Open Patent Application No. 07-261122 has a polarization light splitter located on the light emitting surface side of a parallel light generating device, which is constructed of a light scattering guide that includes a portion having a wedge-shaped profile.
However, these conventional technologies have the following problems. The backlight system of (1) is intended for use with a projection LCD device. Since the structure of the illumination device requires a large amount of space, it cannot be applied for a planar thin LCD device.
The backlight system of (2) is suitable for constructing thin devices. With a polarization light splitter layer fabricated on the inclined sides of a columnar prism array having a plurality of triangular shaped elements, superior efficiency in light usage can be obtained. However, the structure of the polarization light splitter is complicated and, in particular, it is difficult to form the polarization light splitter layer on the inclined sides of the triangular shaped elements. Therefore, the device is not suitable for mass production.
In the technology (3), if the parallel light generating device of the light guide is constructed of a finely designed light scattering guide that includes a portion having a wedge-shaped profile, superior efficiency in light usage can be obtained. However, it is difficult to design and manufacture such a light scattering guide having the desired function. Therefore, this device is not convenient for general use.
Accordingly, the present invention is directed to a backlight device that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a backlight device that has a polarized light splitting film with a relatively simple, thin structure suitable for mass production and that has high efficiency in light usage.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the backlight device of the present invention includes a light source; a light guide for receiving light from the light source at one end and for emitting directional light that has its maximum strength in a first predetermined direction; a polarization light splitter having a planar multi-layer structure; and a beam deflector disposed between the light guide and the polarization light splitter for deflecting the directional light from the light guide light towards the polarization light splitter in a second predetermined direction along which the polarization light splitter has its maximum polarized light splitting effect. As a result, the light usage efficiency from the light source can be improved. In addition, the polarization light splitter of the present invention has a planar shape suitable for manufacturing thin, compact backlight systems and LCD devices.
It is desirable to use a light guide having a plate-like shape. Light diffusing means may be provided in the light guide. Light diffusing means for the light guide may include a light diffusing layer that is in optical contact with the emitting surface of the light guide or formed by roughening the emitting surface or the opposite surface of the light guide. A light guide having light scattering (diffusing) property by itself may also be used as the light diffusing means. In addition, it is desirable that the light deflector be a prism sheet in which the cross-section of the surface facing the light guide and/or the cross-section of the surface opposite to the light guide side have an uneven surface profile formed by a plurality of unit prisms (or unit lenses). Furthermore, it is desirable to have a planar, multi-layer structure for the polarization light splitter in which three or more layers having different refractive indices are laminated. Also, it is desirable that the planar multi-layer structure of the polarization light splitter be constructed of three or more layers such that, with respect to two mutually orthogonal directions in a plane in which optical vibration occurs, the refractive index contrast of adjacent layers in one of the directions and the refractive index contrast of adjacent layers in the other one of the directions are different. Further, it is desirable that the polarization light splitter be constructed of an optical rotation selection layer made of a cholesteric liquid crystal layer and a quarter-wave layer (xcex/4 layer).
An LCD device of the present invention has a structure such that the backlight system described above is used as the rear light source of the liquid crystal cell.
In another aspect, the present invention provides a backlight device including a light source for emitting light; a light guide for receiving the light from the light source and for emitting directional light having a maximum intensity in a first predetermined direction; a polarization light splitter having a planar, multi-layer structure; and a beam deflector, disposed between the light guide and the polarization light splitter, for deflecting the light from the light guide towards the polarization light splitter in a second predetermined direction substantially coinciding with a direction in which the polarization light splitter has a maximum polarization light splitting effect.
In another aspect, the present invention provides a liquid crystal display device including a light source for emitting light; a light guide for receiving the light from the light source and for emitting directional light having a maximum intensity in a first predetermined direction; a polarization light splitter having a planar, multi-layer structure; a beam deflector, disposed between the light guide and the polarization light splitter, for deflecting the light from the light guide towards the polarization light splitter in a second predetermined direction substantially coinciding with a direction in which the polarization light splitter has a maximum polarization light splitting effect such that the deflected light is transmitted through the polarization light splitter; and a liquid crystal cell for receiving the light transmitted through the polarization light splitter to display at least one image.
In a further aspect, the present invention provides a backlight device for use in a flat liquid crystal display device, the backlight device including a light source for emitting light; a light guide for receiving the light from the light source and for emitting directional light having a maximum intensity along a first predetermined direction; a light deflector, disposed over the light guide, for deflecting the directional light from the light guide in a second predetermined direction; and a polarization light splitter, disposed over the light deflector, having a light splitting effect depending on a direction of light incident thereon such that the polarization lights splitter splits the deflected light from the light deflector into two light beams having different polarization conditions with each other, the polarization light splitter reflecting one of the two light beams towards the light guide and transmitting the other one of the two light beams, and the light splitting effect being at a maximum in a direction substantially coinciding with the second predetermined direction.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.